The present invention is directed to nonwoven fabrics and, more particularly, to sheets or strips of nonwovens, including adhesive tapes and the like having nonwoven backings and which are readily and substantially evenly tearable in the CD direction.
The art of manufacturing nonwovens has become a major industry and, in what may be regarded as as highly crowded and competitive art, the patent literature is replete with references thereto.
Without intending to be a complete survey of the art, it can be stated broadly that nonwovens are produced by bonding or entangling natural or synthetic fibers or admixtures thereof.
In one such procedure, the fibers are first carded to orient them primarily in the MD direction, e.g. in a tensile ratio on the order of 9:1, MD to CD. The carded fibers may then be subjected to scrambling after which they may be thermally bonded, e.g. by calendering or point bonding, chemically bonded, or hydroentangled to produce the nonwoven fabric.
In lieu of carding, the fibers may be spunbond or melt blown, followed by airlaying, and then thermally or chemically bonded or hydroentangled.
Irrespective of the method of manufacture it can be said that the nonwovens generally possess much greater tensile strength in the MD direction. Nevertheless, they are not readily tearable uniformly in the CD direction absent some special treatment of the nonwoven fabric to render them so tearable.
High MD strength is a requirement of CD-tearable nonwovens. Techniques which are postulated for promoting CD tear include scoring lines across the fabric to create a weak area which the CD tear would tend to follow; perforating in the cross direction in a similar manner to paper towels; or orienting the fibers in the web predominantly in the CD so that the tear would follow the CD-oriented fibers. However, each of these procedures to facilitate CD tear would materially reduce the potential MD strength and would accordingly not be feasible in products of the type herein contemplated where MD strength is advisable if not an essential requirement.
Other methods of preparing tearable nonwoven sheet materials and/or adhesive tapes made therefrom have been reported in the patent literature. Without intending to constitute a comprehensive survey of the prior art, the following are nevertheless considered to be illustrative of the state of the art.
U.S. Pat. No. 4,233,976 issued to Wayne K. Dunshee discloses a styptic device capable of stopping bleeding in minor wounds and which comrises a porous web of hydrophobic fibers, at least about 50 microns thick, containing a styptic material. A preferred web is said to be a nonwoven web of compacted fibers formed of interlaced staple rayon (or equivalent) textile fibers which is unified by the impregnation with water-insoluble rubbery-fiber-binding/sizing agent that coats the individual fibers without filling the interstices and bonds them together at their crossing points to provide a thin, pliable, inextensible, resilient, water-resistant, porous cloth-like fabric that is said to be strong and tough enough for use in the claimed invention and yet is finger tearable so that the styptic device can be applied from a roll without having to be cut.
U.S. Pat. No. 4,292,360 issued to Riedel et al discloses a multi-ply nonwoven sheet material and pressure-sensitive adhesive tape made therefrom, the nonwoven material comprising a first layer of randomly interlaced fibers bonded to each other by a rewettable binder and at least one additional layer of randomly interlaced fibers bonded to each other and to the first layer by a rewettable binder. The fibers of the additional layer are laid directly on the first layer prior to application of the binder to the additional layer. Various fibers were employed and their tear and other properties described in the illustrative examples and Table 1 which follows. In general, their tear properties in the MD and CD are fairly similar. For instance, with the rayon fabric prepared in Example 1, the MD tear was 1.5 pounds, while the CD tear was 1.0 pounds.
U.S. Pat. No. 4,303,724 issued to Sergeant et al teaches a flexible adhesive tape comprising a pliable base and a low-count open meshed fabric adhered thereto by a layer of adhesive possesses improved tear properties by using a so-called texturized or false-twist yarn in the filling of the fabric.
The task of the present invention can be stated to be to provide nonwoven fabrics and tapes made therefrom which are characterized as being finger tearable substantially evenly in the CD and which are further characterized as having substantially greater tensile strength in the MD.